Fetal Membranes

1. Fetal Membranes Dr Jamila EL Medany

2. OBJECTIVES • By the end of the lecture the student should be able to: • List the components of the fetal membranes. • Describe the stages of development of the components. • Describe the structure and function of the components. • Describe their fate and the possible congenital anomalies.

3. Components • Umbilical cord (Connecting Stalk) • Amnion • Amniotic Fluid • Yolk Sac • Allantois Functions Protection Nutrition Respiration Excretion Synthesis of Hormones

4. Umbilical Cord • It is a soft tortuouscord measuring (30- 90) cm in length (average 55) ,(1-2) cm in diameter. • It is a pathway between the ventral aspect of the embryo and the placenta (chorion) • It has a smooth surface because it is covered by the amnion

5. Structure of Umbilical Cord • 1-Connecting stalk: • Allantois& • Umbilical vessels (two arteries & one vein), they all embedded in • Wharton’s jelly (extra embryonic mesoderm) • 2-Yolk stalk (Vitello-intestinal duct): • A narrow, elongated duct which connects gut to yolk sac • It contains Vitelline Vessels • (Later on , it is obliterated and the vitelline vessels disappear).

6. Normal Attachment of Umbilical Cord • It is attached to a point near the centre of the fetal surface of the placenta

7. Anomalies of Umbilical Cord • (1) Abnormal Attachment: • a-Battledore placenta : • The UC is attached to the margin of the placenta (it is not dangerous). • b-Velamentous insertion of the cord : • UC is attached to the amnion away from placenta, (It is dangerousto the fetus due to liability of rupture of its blood vessels during labor)

8. (2) Abnormalities in Length: • a-Very Long Cord: • It is dangerous , it may prolapse or coil around the fetus. • Prolapsed cord is compressed during labor causing fetal hypoxia or anoxia. • If the deficiency of oxygen persists for more than five minutes , the baby’ brain may be damaged producing mental retardation. • b-Very Short Cord: • It is dangerous because it may cause premature separation of placenta, or the cord itself may rupture

9. (3) knots of umbilical cord:a-False knots: • Normally the UC looks tortuous due to twisting of umbilical vessels (umbilical vessels are longer than the cord), these knots are normal and do not cause any harm to the fetus • b-True knots: • Are rare (1%) of pregnancy, but very dangerous because they may cause obstruction to blood flow in umbilical vessels, leading to fetal death resulting from fetal anoxia. True Knots in 20-weeks fetus

10. Yolk Sac • Its presence is essential for the transfer of nutrients to the embryo during 2nd & 3rd weeks,when the uteroplacental circulation is not established. • It does not contain any yolk. • Its development passes through three stages: • Primary yolk sac. • Secondary yolk sac. • Definitive yolk sac.

11. Primary Yolk Sac Appears in the Blastocyst stage at 10-days, it lies ventral to the embryonic plate. • Its roof :hypoblast(primary endoderm), • Its wall :exocoelomic membrane, (lines the inner surface of the cytotrophoblast),

12. Secondary Yolk Sac • Appears in the chorionic vesicle stage • Its roof :hypoblast . • Its wall :exocoelomic membrane + inner layer (splanchnic layer) of the extraembryonic mesoderm.

13. Definitive Yolk Sac • After folding, part of Yolk Sac is enclosed within the embryo to form the Gut(Foregut, Midgut & Hindgut). • The remainder of Yolk Sac that remains outside the embryo becomes the Definitive Yolk Sac • The midgut is temporarily connected to Definitive Yolk Sac by a narrow duct Vitello-intestinal duct(Yolk stalk),which is incorporated inside the umbilical cord

14. Functions of Yolk Sac • At 3rd week: • (a) Blood formation • first formed in the extra-embryonic mesoderm covering the wall of the yolk sac, until hemopoietic activity begins in the liver during 6thweek (b)Primordial germ cells in the endodermal lining of the wall of caudal end of the yolk sac migrate into the developing sex glands to differentiate into germ cells (spermatogonia or oogonia) • At 4th week:endoderm of yolk sac is incorporated into the embryo as the primitive gut. • Its endoderm gives rise to the epithelium of the Respiratory &Digestive tracts.

15. Fate of Yolk Sac • YolkStalkdetached from midgutby the end of 6th week. • In (2%) of adults, its proximal intra-abdominal part persists as Ileal diverticulum (Meckel’sDiverticulum). • At week (10):small definitive yolk sac lies in the chorionic cavity between amniotic & chorionic sacs • At week (20):, as pregnancy advances, definitive yolk sacatrophies and becomes a very small cyst. • In unusual cases, it persists under the amnion near the attachment of Umbilical cord, on the fetal surface of the placenta. Its persistence is of no significance

16. Allantois • At 3rd week (day 16): • Appears as a diverticulum from caudal wall of YS that extends into the connecting stalk. • At 2nd month:Itsextra- embryonic part degenerates. • At 3rd month: Its intra-embryonic part extends from UB to UC as a thick tube , ‘(Urachus)’ • After birth: the Urachusis obliterated and fibrosed to form Median Umbilical Ligament, that extends from apex of UB to umbilicus.

17. Functions of Allantois • Blood formation • during 3rd-5th weeks. • Its blood vessels persist as the umbilical vein & arteries. • Fluid from amniotic cavity diffused into the umbilical vein to enter the fetal circulation for transfer to the maternal blood.

18. Amnion • It is a thin, transparent & tough fluid-filled, membranous sac surrounding the embryo. • At First : It is seen as a small cavity lying Dorsalto the embryonic plate. • At Stage of Chorionic Vesicle:The amnion becomes separated from the chorion by Chorionic Cavity (extra embryonic coelom). • After Folding: the amnion expands greatly and is becomes on the ventral surface of the embryo. • As a result of expansion of the amnion, the extra embryonic coelom is gradually obliterated and amnion forms the epithelial covering of umbilical cord.

19. Amniotic Fluid • It is a watery fluidinside the amniotic cavity (sac). • It has a major role in fetal growth & development • It increases slowly, to become (700-1000) ml by full term (37) weeks. • Composition: • 99% of amniotic fluid is water • It contains un-dissolved material of desquamated fetal epithelial cells + organic + inorganic salts • As pregnancy advances, composition of amniotic fluid changes as fetal excreta (meconium = fetal feces/& urine) are added

20. Sources of amniotic fluid • Maternal source: • 1-Diffusion acrossamnio-chorionic membrane at the deciduaparietalis. • 2-Diffusion acrosschorionic plate from the maternalblood in the intervillous spaces of the placenta. • Later, it is derived from Fetal source : • Skin, • Fetal Respiratory tract & Mostly by Excreting Urine (at beginning of 11th week)

21. Functions of amniotic fluid • Provides symmetrical external growth of the embryo • Acts as a barrier to infection (it is an aseptic medium) • Permitsnormal fetal lung development • Prevents adherence of embryo to amnion • Protects embryo against external injuries • Keeps the fetal body temperature constant • Allows the embryo to move freely, aiding muscular development in the limbs • Maintain homeostasis of fluids & electrolytes • Permits studies on fetal enzymes, hormones and diagnosis of fetal sex and chromosomal abnormalities

22. Circulation & Fate of amniotic fluid • Amniotic fluid remains constant & in balance • --Most of fluid is swallowed by fetus, and absorbed into fetal respiratory and digestive tracts, whereit is metabolised • --Part of fluid passes through placental membrane into maternal blood capillaries in intervillus space, • Other part of fluid is excreted by fetal kidneys and returned to the amniotic sac through the fetal urinary tract.

23. Anomalies of Volume of Amniotic Fluid • (1) Oligohydramnios: • The volume is less than ½ liters • Causes : • Placental insufficiencywith low placental blood flow • Preterm rupture of amnio-chorionic membrane occurs in 10% of pregnancies • Renal Agenesis(failure of kidney development), • Obstructive Uropathy(urinary tract obstruction) lead to absence of fetal urine (the main source) • Complications : • Fetal abnormalities (pulmonary, hypoplasia, facial & limb defects)

24. (2) Polyhydramnios (Hydramnios): • The volume is more than 2 liters, it is diagnosed by Ultrasonography. • Causes • Fetal( 1-20% ) :Esophageal atresia. • Maternal (2-20%) : Defects in maternal circulation. • Idiopathic(3-60%) • It may be associated with severe anomalies of the CNS

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